An important task of future mobile radio systems is the provision of services at high data rates. In today's digital cellular mobile radio networks of the second generation, for example based on the GSM standard, or of the third generation, for example based on the UMTS standard, the network operators provide their customers with a multiplicity of services. Besides the basic services such as voice telephony, SMS (Short Message Service) and MMS (Multimedia Message Service), video services and IP (Internet Protocol)-based packet data services are also provided.
In view of the success of DSL in the landline domain, the trend in mobile radio is likewise moving toward high-speed mobile radio systems optimized for IP applications (for example VoIP). The current expansion of UMTS with the label HSPA (High Speed Packet Access), also referred to as the 3.5G system, allows maximum net transmission speeds of up to 14 Mbps in the downlink (base station to mobile terminal) and up to 2 Mbps in the uplink (mobile terminal to base station). To ensure that the UMTS system is competitive in the future too, work is currently in progress on the further development of UMTS to produce a mobile radio system which is optimized for IP packet data transmission by improving the system capacity and spectral efficiency.
The aim is to significantly increase the maximum net transmission speeds in future, particularly up to 100 Mbps in the downlink and 50 Mbps in the uplink.
It is expected that at least one transmission direction in the communication system will use OFDM-based signal transmission (Orthogonal Frequency Division Multiplexing). OFDM is a multicarrier method in which block modulation is used to transmit a block with a number of data symbols in parallel on an appropriate number of subcarriers. The sum total of all subcarriers forms what is known as an OFDM symbol for the duration of a data block.
To meet the requirements regarding data rate, however, better utilization of the limited radio resources than is the case in current mobile radio networks is also required. In this respect, systems having a plurality of antennas both on the transmitter and on the receiver provide the option of increasing spectral efficiency through the use of spatial signal characteristics. These developments are covered by the term transmission diversity. At present, discussions are therefore ongoing regarding the use of what are known as MIMO systems (MIMO=Multiple-Input Multiple-Output) in third-generation radio networks and in future WLAN standards (WLAN=Wireless Local Area Network).
An important method for improving the reception conditions and hence possibly increasing the data rate of a communication link is provided by what are known as space-time codes (STCs). It is an aim of a method using these codes to improve the channel characteristics through targeted utilization of spatial diversity by using a plurality of transmission antennas and possibly a plurality of reception antennas. In this case, the use of space-time codes in the downlink of a mobile radio system, for example, is of particular interest, since increased implementation complexity extends only to the base stations and hence it is possible to achieve higher capacity for the downlink of the cellular network and at the same time the implementation complexity for the receivers can be kept down.
One option which has been described for space-time codes is space-time block codes, subsequently also referred to as space-time block coding, for example in the publication “A Simple Transmit Diversity Technique for Wireless Communications” in IEEE Journal on Selected Areas in Communication, 16(8), pages 1451-1458 (1988) by S. M. Alamouti. In space-time block coding, a signal is sent by a first transmission antenna and further transmission antennas send delayed variants of the signal sent by the first transmission antenna. In the permutation scheme, the modulated signal is sent by a first transmission antenna and permutations of the modulated signal are sent by further transmission antennas. Consequently, the signal sent by the transmission antennas can be derived from a matrix which comprises data words in the form of the modulated signal and permutations of the modulated signal. The space-time coding codes a signal into a plurality of data words, and each data word is sent by a different transmission antenna. During sending, the data words are spread over a single carrier frequency in the time domain by sending the data symbols in each data word successively in a single-carrier method.
In the matrix of an STBC, the transmission antennas are usually shown along one axis and the times or timeslots are shown along the other axis. If an STBC data block has T timeslots and codes k data symbols, a code rate r is defined as r=k/T. The space-time block coding was originally introduced for orthogonal STBCs, in which the matrix is such that any two antenna vectors of the matrix are orthogonal with respect to one another. The aforementioned publication by Alamouti describes an orthogonal STBC for two transmission and reception antennas with the code rate 1. In the case of more than two transmission and reception antennas, only orthogonal STBCs for which the code rate is less than 1 are known. Although such STBCs utilize the entire diversity potential of the transmission channel, they do not permit maximum throughput on account of the code rate of less than 1. Besides the orthogonal STBCs, quasi-orthogonal STBCs are also known, in which the relevant vectors are orthogonal with respect to one another only in the case of some of the pairs of antenna vectors. However, it is not known to date how to achieve transmission with a code rate equal to 1 using a quasi-orthogonal STBC when there are fewer than four transmission and reception antennas.
A further development based on an OFDM method relates to a combination of OFDM and MIMO, i.e. sending and receiving via a plurality of paths using a respective plurality of transmission and reception antennas on the stations communicating with one another. The combination of OFDM with MIMO, subsequently also referred to as MIMO-OFDM, advantageously allows the complexity of the space-time signal processing to be reduced.